Abstract: This study aimed to determine whether absolute sex hormone concentrations predict the magnitude of knee joint laxity changes across the menstrual cycle. Twenty-two females (18–30 years, body mass index =30), who reported normal menstrual cycles for the previous 6 months were tested daily across one complete menstrual cycle for serum levels of estradiol (E = pg/mL), progesterone (P = ng/mL), and testosterone (T = ng/dL), and knee joint laxity (KLax = mm displacement at 134N) measured with a standard knee arthrometer. The change in KLax across the cycle (maximum–minimum), and minimum (early follicular) and peak (postovulatory) hormone concentrations were recorded for each subject. A stepwise linear regression determined if the minimum, peak, or absolute change in hormone concentrations would predict the magnitude of change in KLax across the cycle. KLax changed on average 3.2 ± 1.1 mm across the menstrual cycle (range, 1.5–5.3 mm). Minimum levels of E (39.9 ±11.8 pg/mL) and P (0.61 ± 0.27 ng/mL), coupled with peak concentrations of E (199.6 ± 54.9 pg/mL) and T (22.5 ± 10.5 ng/dL) explained 57.6% of the change in KLax across the cycle. Greater absolutechanges in KLax were observed in response to peak E and T levels when minimum E concentrations were lower and minimum P concentrations were higher in the early follicular phase. The absolute minimum concentrations of E and P in the early follicular phase appear to be important factors in determining the sensitivity of the knee joint’s response to changing hormone levels.